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A feeder ramp can dump balls into it. When the elevator is lifted, it can flip back the edge of the ramp, temporarily impounding any oncoming balls. The descending elevator restores the ramp and the flow of balls.

A similar mechanism, used by LegoGBC's Drag & Tilt is to use a single-file ramp in which the balls are held back by a lever. When the elevator crate reaches the bottom, it forces the lever up, allowing the balls to flow into the crate. Simpler than flipping the whole edge of the ramp, it has the downside of requiring a jam-free way of loading the ramp.

trap doors: or suddenly exposed sides. Make one side of the crate a hole, blocked only by the wall of the elevator shaft; at a certain level (perhaps selectable) the shaft has a gap in the wall, spilling the balls. (This is the method used by LegoGBC's Drag & Tilt.)

dumpable: at the top of its travel, the crate is mechanically dumped, possibly by “tripping” on an edge at the top.

cable-based: a string (or perhaps chain links) pull the elevator up and down the shaft

cog-based: either an externally-driven gear cranks up & down a rack on the side of the elevator car (suitable for short drops only, unless you want a long rack sticking out), or the elevator car itself has a motor-powered gear that engages a static rack fixed to the elevator shaft.[1]